1. Field of the Invention
This invention relates to an apparatus for formng thin films such as a barrel type epitaxial growth apparatus used for manufacturing ultra LSIs.
2. Description of the Related Art
With the recent trend for increasing integration density and reducing the size of semiconductor integrated circuits, it is a frequently used practice to form an epitaxial layer by means of epitaxial crystal growth on a semiconductor wafer for MOS, for instance. There is also a trend that the thickness of the epitaxial layer is reduced with increasing size of the semiconductor substrate. Reaction apparatus for the epitaxial growth are roughly classified into three types, i.e., horizontal reaction tube type, vertical bell jar type and barrel type. Further, there is a recently developed hot wall type, which is a modification of the reduced pressure CVD. In these reaction apparatuses, a susceptor for supporting a plurality of wafers is set in a reaction furnace maintained under a high temperature condition, and in this state a reaction fluid, which is a gas such as silicon tetrachloride (SiCl.sub.4) or silane, is supplied into the reaction furnace, whereby an epitaxial layer is formed on each wafer surface.
In a modification of the barrel type epitaxial growth apparatus, each wfaer is heated from two sides for obtaining uniform heating. In this case, the wafer is heated from the back side by a high frequency induction coil through a susceptor, while the bell jar surface is provided with a metal coating film to radiate heat toward the wafer surface.
In order to obtain a high quality epitaxial film, it is important to maintain the thickness and resistance of the film uniform. In the barrel type apparatus noted above, however, it is impossible to control the temperature of each wafer or the temperature of each portion of a wafer constituting each chip uniformly, and non-uniform temperature distribution results. The reason for this is as follows. With the reaction fluid progressively supplied to and discharged from the reaction furnace under a high temperature condition, the temperature of the fluid, i.e., silane, and the molar concentration of a silicon-containing gas and a dopant gas are not the same in the neighborhood of the inlet and outlet of the furnace; that is, the temperature and molar concentration vary depending on the location of the wafer. Further, silane does not flow uniformly through the reaction furnace, but will sometimes form a vortex flow. In such a case, variations of temperature and molar concentration of gas will occur not with different wafers but with different portions of the same wafer. Still further, the amount of heat radiated from the metal coating film formed on the bell jar is reduced with the progress of the epitaxial growth, thus resulting in non-uniform temperature distribution. Furthermore, with the repetition of the epitaxial growth process the metal coating film is deteriorated due to thermal fatigue. Non-uniform temperature distribution is further liable to occur depending on the amount of the supplied reaction fluid.
Further, in considering the amount of processing applied to wafers, with increase of the wafer size the amount processed per cycle is reduced, leading to a cost increase per wafer.